In the art of hot metallic coating of steel tubes by the application of molten zinc or aluminum, one of the problems which exists is the surrounding of the tube with the coating material without the tube being bent to pass through a molten metal bath as has been customary when coating continuous lengths of wire or strip.
It will be realized that in the coating of wires or strips no problem exists as the wire or strip can be bent by passing it over guide rollers to extend down into the molten metal bath and can then be guided along the bath and taken out of the bath at the other end by again passing around appropriately positioned guide rollers. When it is necessary to coat tubing or larger rods or members which cannot be bent, and which have a length such that they cannot be submerged in the bath, the method used for coating is to pass the products through a trough which had closed ends and sides into which the coating liquid is pumped. The trough has apertures in its ends so that the tube can pass into the trough at one end and out at the other with a minimal spill of coating fluid through the apertures.
Processes of the type described are used particularly in the continuous formation of tubes which are then galvanized and cut into lengths. The tubes are formed from flat strip passed through forming guides and bent to tubular form. The joint is then welded to give a continuous tube, means being used to cut off any projecting metal at the weld. The tube is then usually heated by induction in an inert atmosphere and is passed into the galvanizing section in which the trough is positioned above the level of the molten zinc in the kettle. This section is usually also maintained in an inert atmosphere by enclosing the top of the kettle and the trough. On leaving the trough, excess zinc is removed from the tube by means of an air knife or the like surrounding the tube. The excess zinc flows back into the kettle. The tube passes out of this zone to a flying shear which cuts the galvanized tubing to length.
The present invention relates generally to this type of process but is not necessarily limited thereto as it can be applied anywhere where tube or conduit or rod or the like, which will generally be referred to as a "work piece", is required to be continuously passed through a metal coating zone while maintaining linear alignment of the work piece being coated.
Certain objections exist to the use of a trough which has ends and sides and has sealed apertures through which the work piece to be treated must pass. One difficulty is to obtain optimum size of the apertures in relation to the work piece to ensure that, especially at the exit end, the coat of galvanizing material which has been applied to the work piece will not be disturbed or adversely affected. More significantly perhaps, the seals are responsible for wiping the work piece such that the amount of coating matal remaining (0.8 ounces/ft.sup.2 at best) is far less than the prescribed optimum, considered as 1 - 11/4 ounces/ft.sup.2.
A further problem exists in that it is necessary to supply sufficient zinc to the trough to cause the level to be maintained well above the work piece, and to maintain a sufficient flow by pumping excess zinc to the trough to ensure that the level will be maintained and also to ensure that there will be a correct temperature gradient over all parts of the trough for most effective galvanizing.
These and other problems were overcome by my previous inventions as described in U.S. Pat. No. 3,877,975 and U.S. Pat. No. 3,956,537. However, a further problem which exists in processes such as those mentioned above is the continuous formation of "galvanizer's dross" in the case of galvanizing and an aluminum oxide in the case of aluminizing. It has been general practice to enclose the bath as much as possible in an inert atmosphere to prevent the formation of galvanizers dross or aluminum oxide. In the case of galvanizing the dross can appear in three different forms, namely aluminum zinc oxide, iron zinc oxide and an iron zinc alloy. The majority of the iron zinc alloy can be eliminated by replacing the conventional iron kettle with a refractory lined induction heated furnace, and metalizing the other parts of the process which come in contact with the molten metal with a material not wetted by zinc such as molybdenum. In the case of aluminizing, as molten aluminum and iron are incompatible with the aluminum dissolving the iron at a rapid rate, use of the refractory lined furnace is imperative. Aluminum shows no reaction with refractories. These and other techniques were set forth in a technical paper entitled, "Remodeling the In Line Galvanizing Process", which was presented to The Association of Iron and Steel Engineers on May 4, 1976 in Birmingham, Alabama. The oxides, however, are formed through aeration of the molten metal by the air knife, by the excess coating metal falling from the work piece to the main supply and by the exposure to the air of the exit end of the furnace from the purpose of collecting the excess coating metal from the work piece, and the air knife.
Another problem, which exists in processes mentioned above, is the inability to obtain an evenly distributed coating about the periphery of the work piece through the use of an air knife. The force of gravity tends to allow the coating to sag, resulting in a thicker coating about the lower sides and bottom, and a thinner coating on the top and upper sides of the work piece before it enters the air knife for freezing.
These latter deficiencies and other problems are overcome by the present invention.